1. Field of the Invention
The invention relates to current mirror technology and more particularly to current mirror circuits in different ICs sharing the same current source.
2. Description of the Related Art
A current mirror circuit is often used to “mirror” (copy) a current of a current source (reference current) flowing through one transistor to at least one other transistor of the circuit. The current mirror circuit is typically used in equipment that requires current flowing through at least one electronic device to be exactly the same or at least be very close to each other. For example, the current mirror circuit may be utilized in display apparatuses using LEDs (Light Emitting Diodes), OLEDs (Organic Light Emitting Diodes), etc.
FIG. 1 illustrates a conventional PMOS (P-type Metal Oxide Semiconductor) current mirror circuit 10 of the prior art. The current mirror circuit 10 comprises PMOS transistors PM and P1˜Pn. Source terminals of the PMOS transistors PM and P1˜Pn are connected to a voltage source Vdd. A gate terminal (control terminal) and a drain terminal of the PMOS transistor PM and gate terminals of the PMOS transistors P1˜Pn are connected to a constant current source 100 generating a current IC. In the current mirror circuit 10, the PMOS transistors PM and P1˜Pn are assumed to be identical, and thus, output currents I1˜In respectively flowing through the PMOS transistors Pn˜Pn are equal to the current IC flowing through the PMOS transistor PM. However, since threshold voltages Vt and constants β (depending on the transistor dimensions and material used for fabrication) of transistors are not completely identical in practice, the output currents I1˜In are not exactly equal to the current Ic and to each other. The differences in the output currents I1˜In may cause display apparatuses using LEDs or OLEDs to display images unevenly.
The differences may get worse when current mirror circuits in different ICs (Integrated Circuits) share the same current source. FIG. 2 illustrates a block diagram of a semiconductor device 20 comprising PMOS current mirror circuits in different ICs sharing the same current source according to an example of the prior art. The semiconductor device 20 comprises a master circuit 210 and a slave circuit 220. The master circuit 210 and the slave circuit 220 are provided on different ICs. A current mirror circuit 212 in the master circuit 210 and a current mirror circuit 222 in the slave circuit 220 shares the same constant current source 200 in the master circuit 210. The current mirror circuit 212 comprises PMOS transistors PM and P1˜Pn and a current generating circuit 214. The current mirror circuit 222 comprises PMOS transistors PS and P′1˜P′n. The current generating circuit 214 comprises NMOS (N-type Metal Oxide Semiconductor) transistors NT1, NT2 and NT3 and receives a current IC from the constant current source 200. In order to provide the same reference current to the current mirror circuit 212 and the current mirror circuit 222, the current IC of the constant current source 200 is provided to the current mirror circuit 212 and the current mirror circuit 222 through a current mirror structure constructed by the NMOS transistors NT1, NT2 and NT3. A gate terminal and a drain terminal of the NMOS transistor NT1 and gate terminals of the NMOS transistors NT2 and NT3 are connected to the constant current source 200, and source terminals of the NMOS transistors NT1, NT2 and NT3 are connected to a ground end. Thus, the current Ic of the constant current source 200 is mirrored from NMOS transistor NT1 to NMOS transistors NT2 and NT3. A gate terminal and a drain terminal of the PMOS transistor PM and gate terminals of the PMOS transistors P1˜Pn are connected to a drain terminal of the NMOS transistor NT2. A gate terminal and a drain terminal of the PMOS transistor PS and gate terminals of the PMOS transistors P′1˜P′n are connected to a drain terminal of the NMOS transistor NT3. In the semiconductor device 20, the PMOS transistors PM, P1˜Pn, PS and P′1˜P′n are assumed to be identical, and the NMOS transistors NT1, NT2 and NT3 are assumed to be identical. Thus, output currents I1˜In and I′1˜I′n are all equal to the current IC. However, since threshold voltages Vt and constants β of transistors in an IC are not completely identical in practice, even though the current IC is mirrored to the current mirror circuit 212 and the current mirror circuit 222 in different ICs, output currents between ICs may not be completely identical.